This invention relates to the connector-cable assemblies such as are used to make multiple interconnections between high speed circuits in computer, and similar electronic equipment.
Many present day electronic circuits (semiconductors, large scale integrated circuits, etc.) have much higher densities, and faster switching speeds, than circuits of only five years ago. These modern circuits produce signal pulses with nano second, or even sub-nanosecond rise times, and relatively low power. Where it is necessary to transmit with high integrity the signals from one circuit to another that is physically removed by some distance (e.g. five feet), present day practice frequently is to use a flat cable with multiple signal lines. Each line of the cable has an impedance that is closely matched to the impedance of the circuits it is interconnecting. This impedance matching is necessary to prevent undue amount of distortion, of attenuation, and of cross-talk of the low power electronic signals traveling along the line in the cable. Cables with impedances in the range from 50 to 95 ohms, and with from eight to forty signal lines are commonly used.
For reasons of mechanical, thermal and electronic performance, and also because of efficient size and installed cost advantage, a widely used type of matched impedance cable comprises a thin, flat ribbon of tough, low loss insulation, such as Teflon (Du Pont trademark). Buried in the insulation are many fine gage, closely spaced wires which serve as multiple transmission lines. The wires are usually arranged in triplets in which a center signal wire is closely paralleled on each side by a ground wire. The impedance of each "triplet" transmission line is determined by the effective dielectric constant of the insulation surrounding them, by the gage of the wires, and by their distance apart.
By way of example, and as an aid in understanding the invention described hereinafter, one such flat cable, which is widely used, comprises a thin ribbon of PTFE Teflon in which are buried sixty-six plated copper wires of 32 gage each. The wires are arranged in twenty-two "triplets" with all of the wires being evenly spaced and with a nominal 50 mil (50 thousandths of an inch) center-to-center spacing from signal wire to signal wire. The cable insulation is about 30 mils thick and 1.13 inch wide. The impedance of each triplet line is nominally the same ohmage (plus or minus a few percentage points because of manufacturing tolerances). This impedance is measured with sub-nanosecond rise-time pulses, which showed a propagation delay of slightly under 1.4 nanosecond per foot along ten feet of the cable.
In the inventor's previous patents, U.S. Pat. No. 4,173,388 and U.S. Pat. No. 4,288,917, there are described a contact device for, and a method of, mechanically terminating the fine gage wires of a flat cable. Very high reliability of the wire terminations made according to these prior patents was obtained. But various additional design considerations led to manufacturing and assembly complexities in changing to a different cable-connector combination in which there were many more wires, on much closer centers, and with a different "ground" and "signal" pattern for the output sockets.
One of the problems involved in designing a high performance electronic connector is how to provide output contacts which can repeatedly be plugged into or unplugged from the input-output (I-O) contacts of a circuit without undue mechanical wear or degradation of the electrical interface between connector and circuit. Typically, the I-O contacts of a circuit are 25 mil square "wire-wrap" posts on closely spaced columns and rows (e.g. tenth inch by tenth inch). These posts mate with connector contacts which include spring members that grip the posts and directly or indirectly, provide electrical connection. Because a connector may contain up to several dozen output contacts it is highly desirable that the insertion force of each contact onto a post be kept relatively low (e.g. several ounces). But the spring force holding a contact against a post must remain throughout its lifetime above the minimum force needed for a reliable, low resistance electrical connection. Even where gold is used at the interface of contact to post, these mechanical and electrical requirements are hard to meet. An excellent discussion of various important requirements of high performance output contacts in electronic connectors for critcal circuit application is given in an article titled "PRINTED-CIRCUIT-BOARD CONNECTOR FAMILY WITH UP TO FORTY-EIGHT CONTACTS PER INCH OF BOARD HEIGHT" by C. L. Winings of Bell Telephone Laboratories, published in the 1980 Proceedings, 30th Electronic Components Conference, pages 332 to 340.
One of the best ouput contacts for an electronic connector comprises a miniature box-like socket which plugs onto a 25 mil square I-O contact. Mounted within the body of the socket is a very small separate leaf spring which when the socket is on the post holds the body of the socket against the post with a controlled force. By choosing a suitable metal and appropriate geometry for the spring, the insertion force of the socket onto a post and its holding force when on the post are closely controlled to desired values. The body of the socket generally is formed from a metal which is easier to bend and less expensive than the metal of the separate spring. A high performance socket of this kind is shown in U.S. Pat. No. 3,370,265 to Berg.
Because the sequence or pattern of electrical "ground" and "signal" I-O contacts may vary from circuit to circuit, it is necessary to provide different patterns of ground and signal outputs for the connectors used with these circuits. Where the connector contacts are numerous and closely spaced in multiple rows, changing from one wiring pattern to another has previously been expensive and involved considerable "hand" labor in assembly. The present invention seeks to overcome these prior difficulties. This invention is highly useful in, but not limited to the connector arrangement described and claimed in the inventor's co-pending application Ser. No. 633,897, filed on even date herewith titled FLAT CABLE-CONNECTOR HAVING IMPROVED CONTACT SYSTEM.
It is an object of this invention to provide an electronic connector having multiple, high performance output sockets, the "ground" and "signal" pattern of the sockets being easily and inexpensively changed.
A further object of the invention is to provide an improved electronic connector which is also less expensive to manufacture and to assemble than previous similar connectors.
Still another object is to provide a manufacturing technique or method for electronic connectors which is highly versatile and inexpensive and yet gives precision and uniformity to the many critcal parts in the connector.